Avalanche photodiodes (APDs) are solid-state photodetectors in which a high bias voltage is applied to a p-n junction to provide a high first stage gain due to avalanche multiplication. Avalanche multiplication occurs when an incident photon with sufficient energy to liberate an electron arrives at the photodiode. The high electric field rapidly accelerates the photo-generated electron towards the anode, but before it can reach the anode, it collides with the intervening doped material releasing further electrons, all of which are then accelerated towards the anode. This process repeats leading to avalanche multiplication of the photo-generated electron and an output current pulse. APDs are thus semiconductor analogs to photomultiplier tubes. Linear mode APDs are effectively single stage linear amplifiers in which the gain is set by controlling the bias voltage, and with gain factors of several hundred can be achieved in linear mode.
Single-Photon Avalanche Diodes (SPADs) are APDs in which the p-n junction is biased above its breakdown voltage to operate in Geiger mode such that a single incident photon will trigger an ongoing avalanche breakdown and thus easily measurable current pulse. That is a SPAD operates as a trigger device generating a large current pulse compared to linear mode APDs in which the current pulses can be very low at low light intensity. After triggering of the SPAD, a quenching circuit is used to reduce the bias voltage below the breakdown voltage in order to quench the avalanche process. Once quenched the bias voltage is again raised above the breakdown voltage to reset the SPAD for detection of another photon (known as re-biasing the SPAD).
APD and SPADs are solid state devices and can be constructed using a variety of CMOS technologies, and have very small active areas compared with other photon counting devices such as photomultiplier tubes. Through appropriate choice of materials and structure, wavelength sensitivity of a SPAD can be controlled to be in the visible and/or near-infrared range. A SPAD combined with additional circuitry to count pulses and/or measure time of arrival of photons to sub-nano/pico second accuracy can be used to create sensors for ultra-low light imaging or highly sensitive time-resolved imaging applications. For example one potential application of a SPAD arrays are in three dimensional (3D) Flash LIDAR cameras, as they have the potential to provide extremely sensitive devices with high distance resolution and high frame rates. SPAD arrays also have potential for use in other applications that require single photo sensitivity with high frames such as biological/medical imaging applications, adaptive optics applications, and astrophysics applications.
Three dimensional Flash LIDAR systems, also known as 3D Time of Flight (TOF) Cameras, use a laser source to irradiate a target with a short duration laser pulse (ie a laser flash). Photons are back scattered off objects and onto the sensor and the time of arrival is used to determine time of flight and thus distance to an object. The first 3D TOF cameras were constructed as scanning laser systems comprising a laser range finder with a rotating or scanning element(s) to progressively scan the field of view. These scanning systems are effectively single pixel devices collecting time of flight information in a single direction which build up a 3D image progressively moving the pointing direction of the sensing element. More recently scanner-less (ie staring) 3D Flash LIDAR systems have been developed using a two dimensional array of linear mode APDs (pixels) to achieve more rapid scene capture than scanning systems, and to avoid problems with scanning systems such as mechanical wear, vibration, and/or motion blur. By rapidly resetting APDs after triggering, each pixel can be used to receive multiple photons from the laser pulse, each corresponding to different distance. In this way, a 3D point cloud of the target scene can be rapidly generated.
SPAD arrays in 3D Flash LIDAR systems have the potential to achieve even greater light sensitivity with high distance resolution and high frame rates than scanning laser systems or scanner-less APD based systems. However, achieving high spatial resolution with a SPAD array has proved challenging and most SPAD arrays are experimental with only a relatively small number of pixels (eg 6×8, 32×32). In particular SPADs require the use of very high voltages, and design of efficient SPAD structure is a difficult problem. Further each SPAD requires an adjacent quenching circuit and a triggering detection (ie time of arrival) circuit, all of which takes up additional space on the substrate immediately surrounding the SPAD.
There is thus a need to provide improvements to allow construction of high density SPAD arrays, or at least provide a useful alternative to current SPAD array systems.